A multilayer laminated film composed of a number of layers having low refractive index and a number of layers having high refractive index alternately laminated one upon another may form an optical interference film selectively reflecting or transmitting light of a specific wavelength by the structural light interference between the layers. Such multilayer laminated film may form a film having excellent design by the structural color development, for example a pearlescent film having iridescent appearance by setting the wavelength of the selectively reflecting or transmitting light in a visible light range. The obtained design is based on the structural color development of the multilayer laminate film and, accordingly, there is no color fading trouble in contrast with the color developed by a dye, etc. A high reflectance comparable to that of a film produced by using a metal can be attained by the multilayer laminated film by gradually changing the film thickness or laminating films having different reflection peaks, and such film can be used as a metallic luster film or a reflecting mirror.
Multilayer laminated films produced by using different kinds of thermoplastic resins such as polyethylene terephthalate and polymethyl methacrylate have been proposed as such multilayer laminated film for example in the specification of the JP-A 56-99307 (hereunder JP-A means “Japanese Unexamined Patent Publication”). Multilayer drawn films produced by using a layer of polyethylene 2,6-naphthalenedicarboxylate as the layer having high refractive index were proposed in the specifications of JP-C 9-506837 (hereunder JP-C means published Japanese translations of PCT international publication for patent applications) and WO01/47711.
However, these multilayer laminated films have poor adhesion between the layers to occasionally cause the delamination defect because resins having different compositions are combined with each other in these films to increase the refractive index difference between the layers. Furthermore, the uniform drawing becomes difficult to form an easily tearable film having insufficient strength by the combination of resins having different compositions.
Namely, the refractive index difference between the layers of conventional multilayer drawn film was originated from the refractive index difference of the resins constituting the layers. For example, polyethylene terephthalate was used in the layer having high refractive index and a resin having low refractive index such as polymethacrylate was used in the layer having low refractive index as disclosed in JP-A 56-99307. The concept of the conventional method to get the refractive index difference between layers by the refractive indices of the resins necessitates the selection of resins having considerably different compositions as the resins constituting the layers and the adhesivity between layers becomes poor.
Examples shown in JP-C 9-506837 and WO01/4771 contain a biaxially drawn film produced by using a polyethylene 2,6-naphthalenedicarboxylate (hereunder sometimes referred to as PEN) having high refractive index as the layer having high refractive index and a thermoplastic elastomer as the layer having low refractive index and a uniaxially drawn multilayer film produced by using PEN having high refractive index as the layer having high refractive index and a PEN copolymerized with 30 mol % isophthalic acid as the layer having low refractive index. The layer having low refractive index is essentially amorphous in these multilayer laminated film and these films have latent practical problems to fail in getting a sufficient interlayer adhesion even by the drawing treatment and in achieving uniform biaxial drawing in the plane directions to cause non-uniform properties.